It is well known to apply an electroless metal plating to an insulating substrate such as a polymer. Generally, the process involves depositing a noble metal, such as platinum, gold or paladium which acts as an initiator or catalyst for the electroless metal plating onto the surface of the insulating substrate, and then dipping the catalyzed substrate into an electroless metal plating solution. It is also known to catalyze a surface with a non-noble metal catalytic layer, e.g., Cu, by reduction of a copper compound absorbed on the surface.
Various techniques have been proposed to form a circuit on the insulating substrate by electroless metal plating for the manufacture of printed circuits on flexible or rigid substrates. The most common methods employed may be divided into two catagories, subtractive processing and additive processing.
In the subtractive method for forming a circuit pattern on a substrate, the substrate employed has a thin metal layer laminated to its surface. A photoresist pattern is then formed over the metal laminate and the laminate is then treated with an etching solution so as to etch the exposed portions of the metal laminate, leaving metal only in the area protected by the photoresist. This method has several disadvantages among which is the necessity for the use of relatively large quantities of etching solution, undercutting of the metal during etching which prevents formation of high resolution patterns and the waste of base metal which must be etched away and either reclaimed or disgarded. Electroless plating is employed as an adjunct to the subtractive processing when one requires throughholes in the printed circuit board for the mounting of devices or interconnection of circuit patterns from one layer of the board to another. These throughholes must be metalized to insure circuit continuity and/or good adhesion of inserted devices by soldering.
One additive electroless technique involves first sensitizing and activating the entire substrate surface before putting down a resist pattern and then after putting down the resist pattern, the entire surface is treated with the electroless metal deposition solution. In this method metal deposits only on the sensitized and activated exposed substrate areas and not on the resist-covered areas. Resolution is quite good in this method, however, this method has several disadvantages: e.g., (1) sensitization of the substrate surface produces a surface which may have a relatively low resistivity between the deposited conductors (if spacing is to be very close between conductors, as is required in many of todays high density applications, this may cause electrical breakdown); and (2) manufacturing handling problems exist in that the catalytic surface is highly sensitive to contamination, scratching and the like which can result in defective circuits.
An improved additive processing techique was described by Sawyer in U.S. Pat. No. 4,388,351. In accordance with this techique, a negative mask is applied to the surface of the substrate whereby portions of the surface are exposed in a positive manner. Micropores are formed in the positive pattern portions of the substrate surface such as by chemical treatment or sputter etching. The positive pattern portions of the substrate and the negative mask are then sensitized to form catalytic species thereon capable of catalyzing an electroless metal deposition. A thin/porous flash metal deposit having a thickness of, for example, from 0.003 to 0.02 inches is deposited on the delineated catalytic species. The negative mask is then removed, thereby removing the catalytic species and flash electroless metal deposit thereon. This leaves the catalytic species and flash deposit in the desired pattern on the substrate surface. The conductive pattern is built up by electroless deposition onto the flash deposit to a desired thickness.
Generally, in the past, most photoresists used for defining the required circuit patterns have been of the type which require chlorinated hydrocarbon solvents to develop the resist pattern. In recent years, aqueous photoresists have been a subject of intense interest in the printed circuit industry. The most desirable feature of an aqueous photoresist process is that aqueous alkaline solutions are used to develop and strip the resist rather than the chlorinated hydrocarbon solvents which present difficult environmental and health problems. It would therefore, be advantageous to be able to employ an aqueous photoresist in the catalyst lift-off process just mentioned. However, the mere substitution of an aqueous photoresist for non-aqueous photoresist was found to be non-trivial. It was discovered that when the same chemistry was employed in the process as described by Sawyer, except for substitution of the aqueous photoresist for the non-aqueous photoresist, the aqueous photoresist was often undesirably prematurely stripped from the substrate during treatment and further, contaminated the electroless plating bath. We have now discovered that by changing some of the chemical solutions previously employed in the catalyst lift-off process, the integrity of the aqueous photoresist can be maintained and a continuous low cost commercial process has become feasible.